The first and the second critical micelle concentration (CMC(1) and CMC(2)) for three alkyltrimethylammonium bromide (C(n)TAB)/sodium dodecylsulfonate (AS)/H(2)O mixed systems, and CMC(1) for trimethylene-1,3-bis(dodecyldimethylammonium bromide) (12-3-12)/AS/H(2)O mixed system have been measured. The largest negative β(m) value means the strongest synergism between 12-3-12 and AS. The CMC(1) and CMC(2) for the C(n)TAB/AS/H(2)O mixed systems decrease with the increase of n. The equimolar mixed systems give the smallest CMC(1) values, whereas the CMC(2) values decrease with the increase of the composition of the surfactant with higher surface activity in the C(n)TAB/AS/H(2)O mixed systems. For the C(16)TAB/AS mixed systems far from equimolar, specific counterion effect on lowering CMC(1) enhances according to the Hofmeister series. There is slightly or no salt effect on the CMC(1) of the other wide composition range of C(16)TAB/AS/H(2)O mixed system. The pseudophase separation model coupled with the dissociated Margules model has been proposed and gives satisfactory description of the mixed CMC(1), the calculated micellar compositions are in well accordance with composition information from the ζ potential measurements. The addition of salt into the C(16)TAB/AS/H(2)O mixed system, leads to a certain degree of decrease in CMC(2). In addition to counterion effect, the co-ion effect on CMC(2) of the mixed system was explained using Collins' concept of matching water affinities.

Phase inversion of aqueous two-phase systems with excess cationic surfactant (abbreviated as ATPS-C) formed by aqueous mixtures of 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (abbreviated as 12-3-12) and sodium dodecyl sulfonate (abbreviated as AS) at 318.15 K was investigated. The experimental results indicate that addition of NaF, NaCl, NaHCO 3, or NaNO 3 can result in phase inversion of ATPS-C formed by 12-3-12/AS systems; however, addition of NaBr cannot lead to phase inversion. TEM micrographic experiments illustrate that there is no direct relationship between the microstructures of the concentrated phase in ATPS-C and phase inversion. To interpret the phase-inversion phenomena of ATPS-C, the phase composition, phase density, and phase volume ratio between the dilute phase and the concentrated phase in ATPS-C were investigated. Phase composition analysis results illustrate that for the ATPS-C formed by 0.10 mol.kg (-1) 12-3-12/AS mixed system, the concentration of Br (-) counterions in the dilute phase of ATPS-C increases with addition of NaF, NaCl, NaHCO 3, or NaNO 3. At the same time, the molar ratio between the F (-) (Cl (-), HCO 3 (-), or NO 3 (-)) counterions and Br (-) counterions in the concentrated phase of ATPS-C increases also. It illustrates that part of the bromide counterions which are the natural counterions of the surfactant 12-3-12 in excess are exchanged by other anionic counterions when an additional salt is added to the system. The investigation indicates that the common ground of the added F (-), Cl (-), HCO 3 (-), or NO 3 (-) counterions is that they all make a smaller density contribution than that of Br (-) counterions, although they have a weaker or stronger counterion binding ability with the mixed positively charged aggregates in ATPS-C than that of Br (-) counterion. Density experiments illustrate that the density increase of the dilute phase is larger than that of the concentrated phase in the ATPS-C with addition of NaF, NaCl, NaHCO 3, or NaNO 3; thus, phase inversion occurs. The densities of the added inorganic sodium salt aqueous solution and the order of the Hofmeister series for the added inorganic anions with respect to the chaotropic headgroup of 12-3-12 play important roles in the phase inversion of ATPS-C.

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